A judge dropped one of four charges Monday against Lt. Brian Rice in the arrest and death of Freddie Gray.

A judge dropped one of four charges Monday against Baltimore police Lt. Brian Rice in the arrest and death of Freddie Gray, ruling prosecutors had not presented enough evidence to prove second-degree assault after three days of witness testimony.

Circuit Judge Barry G. Williams made the decision at the trial's midpoint — after the prosecution rested its case, but before Rice's attorneys launched into his defense — by partially granting a defense motion for acquittal. He denied the motion as it pertained to involuntary manslaughter, reckless endangerment and misconduct charges, though he said his decision on the reckless endangerment count was an "extremely close call."

Such motions are seldom granted, in part because judges have to consider the charges in the "light most favorable to the state," as Williams noted. But in this case, he said, he was "simply not satisfied" that prosecutors had proved assault based on their theory of how it occurred.

That theory, according to an initial outline of the charge cited by Williams, is that the police transport van in which Rice placed Gray became an "instrumentality" of Rice's when Gray suffered fatal spinal cord injuries in its rear compartment last year.

Williams questioned how the van could be an "instrumentality" of Rice's if he wasn't the one driving, and said there had been no evidence that Rice had been acting "in concert" with others, as the theory also held.

Legal observers in the courtroom said the decision to toss the charge came as a surprise.

David Jaros, a University of Baltimore law professor, said he felt that "the fact that the van driver was following [Rice's] orders on where to go with the van" was enough to allow the charge to move forward, even though he has "significant doubts as to whether [the prosecution] can prove their case beyond a reasonable doubt" in the end.

Douglas Colbert, a University of Maryland law professor, said that if one believes, in the light most favorable to the state, that Rice left Gray "in this dangerous, precarious position, and he had no way to protect against an injury, a serious injury, and one occurred thereafter," then it would "seem to be enough to hold the charge" through the conclusion of the trial.

Photos from the trial of Lt. Brian Rice, the highest-ranking Baltimore police officer charged in the arrest and death of Freddie Gray.

"But the judge is saying the evidence is not convincing," he said.

Gray, 25, died in April 2015 after suffering severe spinal cord injuries in the van, prosecutors say. His death sparked widespread protests against police brutality, and his funeral was followed by rioting, looting and arson.

Rice, 42, the highest-ranking of six officers charged in the case, originally faced five charges. Prosecutors dismissed one — a second misconduct-in-office charge — at the start of his trial.

Prosecutors allege Rice was negligent and uncaring when he helped place a handcuffed and shackled Gray on the floor of the van's rear compartment without securing him with a seat belt. Chief Deputy State's Attorney Michael Schatzow has said that decision cost Gray his life.

Rice's defense attorneys have said their client made the decision quickly during a volatile situation, and that his actions were "100 percent reasonable" given Gray's combativeness and the safety risks associated with belting a detainee in a cramped van.

Rice is the fourth officer to stand trial in the case. The first trial, of Officer William Porter, ended in a hung jury and mistrial in December. Officers Edward Nero and Caesar Goodson Jr. were acquitted of all charges in bench trials in May and June, respectively.

The prosecution's case against Rice was shorter than its cases against the other officers, in part because both sides agreed to the admission of certain evidence and testimony presented in the previous trials.

Prosecutors called 12 witnesses, including three on Monday. Two of them were Rice's co-defendants — Nero and Porter.

Schatzow immediately sought to cast both officers as hostile witnesses. He cited defamation lawsuits they have filed against Baltimore State's Attorney Marilyn J. Mosby and said they had been cooperating with Rice's defense — which Porter later admitted.

Prosecutors tried to use the officers' testimony to establish a narrative of what happened at various stops of the van, including that Rice never secured Gray in a seat belt.

"Lieutenant Rice had no conversation with you about whether to seat-belt Mr. Gray or not, did he?" Schatzow asked Nero.

"No," Nero said.

On cross-examination, both officers bolstered the defense case. Nero, in particular, backed the idea that Rice made the decision not to secure Gray with a seat belt in part because he and other officers felt outnumbered as onlookers yelled at, filmed and "closed in" on them.

"There wasn't enough of us," Nero said.

"And was that a cause of concern for you?" asked Rice's attorney, Michael Belsky.

"Absolutely," Nero said.

Jaros said later that the two officers clearly are "a double-edged sword" for prosecutors.

"They need them to establish certain basic facts about what happened, but they are going to offer all kinds of contextual evidence that favors the defense. ...The prosecutor simply doesn't have a choice, because they don't have other sources to bring in that evidence."

The defense began its case Monday afternoon by calling two witnesses: Donta Allen, the arrestee placed on the other side of a metal partition from Gray in the back of the van, and Officer Zachary Novak, an officer who responded to the arrest scene and was given immunity by prosecutors to testify before the grand jury that indicted Rice and the other officers.

Allen testified that he heard banging from Gray's side of the partition that sounded like Gray's "head was hitting the metal." Novak testified that the scene at the van's second stop, where Rice loaded Gray into the van, was volatile — though Deputy State's Attorney Janice Bledsoe challenged that testimony by playing city surveillance footage that showed fewer people at the scene than he testified were there.

Colbert said the testimony of Nero, Porter and Novak showed what many people in Baltimore fear about the prosecution of police: that officers protect one another on the witness stand.

"Virtually every police witness called by the prosecution is trying to be as much help to the defense as it is to the prosecution," he said.